Zinn suggests that one reason early reason why Columbus oppressed indigenous people was out of fear of "the other." In landing in what he thought to be India, Columbus initially feared those who were there, believing that they stood in the way of his conquest and eventual glory. It is in this where the subsequent motive for oppression happened. This became the drive for wealth and the desire to control. Columbus understood that he could gain more money and accumulation of wealth if he took the natural resources he found, including the people he found there. The emergence of enslavement and oppression was driven by material reality. This desire for material reality was subsequent to the initial motive which was fear of a new element.
The combination of initial fear and subsequent coveting of wealth helped to establish the narrative of Columbus' oppression of the indigenous people. It is in this regard that Zinn introduces that a significant part of the American narrative involved the need to dominate and control others to substantiate individual gain.
There are many reasons why Columbus enslaved the native people. The first reason was because, as he wrote in his journal, they would be good servants. Columbus admitted the native people had done nothing wrong. However, their labor would benefit Columbus tremendously. This brings us to additional motives. Since Columbus got a share of the profit, the more minerals he could mine with the enslavement of the native people, the more money he would get which in turn would allow Spain to profit more. As long as Spain was profiting, the King and Queen would continue to have an interest in sponsoring his voyages to the New World. Additionally, by enslaving the native people, he could convert them to Christianity. This was at the time Spain was expelling the Jews from the country, so increasing converts to Christianity would also please the King and Queen. Even though Columbus felt the native people had done nothing wrong, Columbus knew he would gain personally by their enslavement as would the country of Spain.
Columbus was motivated to find gold where he landed, and to acquire this information he had to use force against some of the natives in order to obtain it. The first thing he noted after his initial encounter with the Indians is that they were generous and peaceful but ignorant people. He realized that it would be difficult for the Indians to resist, since they had inferior weapons and were generally naïve, according to him.
Columbus required help from the royal family and to motivate them, he promised them gold and slaves. He received the needed support in the form of ships and men. On his next voyage, he raided villages and took the inhabitants as slaves. Word went round among the natives about the intentions of the Europeans; as a result, they left their homes.
In summary, the motivation was first to find gold for purposes of wealth generation and second to get slaves in order to earn support from the monarchy.